It is known in the art relating to internal combustion engines that the cylinder charging characteristics of a multi-cylinder engine may be enhanced in one or more portions of its speed range by providing a tuned induction system and/or manifold for conducting the intake air or charge to the engine cylinders. The location and speed range of enhanced charging of a tuned intake system is dependent upon a number of factors which are generally known in the art and thus will not be discussed at length in this disclosure. These factors include, among others, the size and number of plenums, the length of plenum to cylinder runners and the length of intake, or zip, tubes leading to the plenum or plenums.
In a known arrangement where a single plenum is connected to all cylinders of a multi-cylinder engine, charge enhancing resonance is strongest (reaching a peak) toward the higher end of the engine speed range. An alternative arrangement using dual plenums, each connected with half the engine cylinders, and joined by zip tubes to a common air cleaner chamber, provides tuning that strongly enhances the charging effect in the lower engine speed range but drops off in the higher range to a level lower than that of the single plenum manifold. If the advantages of both arrangements could be incorporated in a single system, improved charging efficiency and engine performance could be provided over all, or a greater portion, of the engine speed range.